Armored Core 6 is pretty easy to 100% and it's really enjoyable the whole way there. Just don't bother with PvP.

And my favorite 100% grinding game that I keep playing just because I enjoy it so much, is the Earth Defense Force series. The game encourages you to play each mission with each of the 4 classes in each of the 5 difficulty levels. Even though clearing Hard also gives you Easy and Normal completion too, you're still looking at over 200 hours of gameplay, easily. The games have over 100 missions, tons of weapons to gradually improve, and at higher levels become true combat puzzles to solve with said weapons. Cheese is a way of life, in an enjoyable way.

Dark Souls 1, Dark Souls 2 but not Dark Souls 3. The silver knight farm is brutal and I'd never do it again.

The only game I've ever done 100% Mario 64

Citizen Sleeper, Chants of Sennaar, Papers Please, The Curse of the Golden Idol.

Escape velocity (1996, ambrosia software game for macOS). My favorite computer game of all time I think.

Baldur's Gate 1 & 2...the exploration of the wilds while so vulnerable in the first game is something I'll never get bored of while the second game just takes it to a whole other level with your power growing so immense. They still totally hold up with the extended editions!

South Park: The Stick of Truth. You can 100% it in one run and it takes around 20 hours. Super fun game too :)

FF7, I replay the game every 4-5 years.

Bioshock trilogy

Only games I ever 100%ed were....Assassins Creed 1 and Rocket League, strictly by achievements only. Like I never made it past Champ in RL, and AC1 was really just beat game and collect flags. I never have the urge to 100% a game cause usually it's just a grind for grinds sake and that isn't fun to me

If you mean 100% achievements on Steam for example, I really enjoyed doing that with the following games:

• Slipstream (2018): arcade racing game, 7.5h to 100%

• SpongeBob: Battle for Bikini Bottom - Rehydrated (2020): platformer with some collectathon elements, 13h to 100%

• Polyball (2017): think Super Monkey Ball, but a bit faster and stronger momentum iirc, gets quite difficult later on, and although the amount of content isn't ridiculous, it's very very hard to manage the requirements for 100%. My playtime is 59h, but I kept playing after 100% to get into the top 10 leaderboards on a bunch of levels, so I think it was more like 30-40h for 100%.

• The Stanley Parable (2013): narrative game with some unconventional puzzle elements, 40h to 100%, but not really: one of the achievements is "play the game for the entirety of a Tuesday", so that adds over 24h. Another achievement is to not play the game at all for 5 years. Some people love this silly stuff, some hate it, up to you :)

• Firewatch (2016): narrative game with exploration and some puzzles, 6h to 100%.

Bully

Kotor.

Pokemon Uranium. The (in)famous(?) banned fan game that is still being developed. Minus a couple pokemon, it follows the Black and White philosophy of having no returning pokemon.

I've played through the game multiple times, beating the final fight for the main story and calling it there, without hating myself or the game.

Hell, I've even gone through the trouble of completing the pokedex once and struggled to find an early route bird for the last two entries I needed. Pokedex for that game can be hard because when it comes to 2 of the pokemon, you can only get one per save and must online trade to get the counterpart and 1 line is a trade evolution with no in game trades for it or trade evo items. Obviously starters require online trade, too.

Only problem I have with it is the fact I'm still waiting for the rest of the post game to be developed and released so I can hopefully get the unreleased legendaries you can find on the fandom wiki.

Most Lego games. It's like a physical pain to me, NOT to 100 % them.

Two games I have platinumed on PS4 that I still go back and play are Subnautica and Kerbal Space Program. Not sure if you mean 100% as in like scan and collect every item, or get all the trophies, cause I don't think I've ever 100% a game in that way.

Super Mario World (I 100% it multiple times)

New Super Mario Bros Wii (I 100% with friends and we still play it time to time via Dolphin Online)

Mario 64 (Also did 100% multiple times)

Mario Galaxy 1 and 2

Super Mario Bros Wonders (I also 100% two times already)

Super Mario Odyssey

Super Mario 3D World + Bowsers Furry

Super Mario Sunshine defiantly NOT!

HOLLY MOLLY to 100% this game you must be a real masochist!

Besides Mario Games: Zelda Majoras Mask, I love this game

Ocarina of Time

Elden Ring (I 100% The Base Game a few times, once Singleplayer a few other times with Seamless Co-op)

It was tough, but I had an absolute blast 100%'ing Neon White and Hades

Sekiro